ActinideElement 95 · f-block

Americium (Am)

Americium (Am) is an actinide with atomic number 95; its most stable known isotope has mass number 243. At room temperature it is a solid that melts at 1,176 °C.

95AmAmericium[243]

Atomic number

protons

Atomic weight

[243]

most stable isotope

Melting point

1,176 °C

1,449.15 K

Boiling point

—

Density

12 g/cm³

near room temperature

Electronegativity

1.3

Pauling scale

State at 25 °C

Solid

Discovered

1944

United States (Chicago)

Americium at a glance

Identity

Symbol	Am
Atomic number (Z)	95
Atomic weight	[243] mass number of the most stable isotope
Category	Actinide
Group	— (f-block: actinide series)
Period	7
Block	f-block
CAS number	7440-35-9

Atomic properties

Electron configuration	[Rn] 5f⁷ 7s² Full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁷ 6s² 6p⁶ 7s²
Electrons per shell	2, 8, 18, 32, 25, 8, 2
Oxidation states	+3 Less common: +2, +4, +5, +6, +7
Electronegativity (Pauling)	1.3
First ionisation energy	5.974 eV (576.4 kJ/mol) 2nd: 11.7 eV · 3rd: 21.7 eV
Atomic radius (empirical)	175 pm
Covalent radius	166 pm
Van der Waals radius	244 pm

Physical properties

State at 25 °C	Solid
Melting point	1,176 °C · 2,148.8 °F · 1,449.15 K
Density	12 g/cm³

Isotopes & abundance

Natural occurrence	Synthetic
Radioactive	Yes
Most stable isotope	²⁴³Am — half-life 7.35 thousand years
Known isotopes	27

Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.

Americium electron configuration

Americium has 95 electrons. Written in short form, its ground-state configuration is [Rn] 5f⁷ 7s² — the electrons of the noble gas radon, plus 5f⁷ 7s².Written out in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁷ 6s² 6p⁶ 7s².

5f↑↑↑↑↑↑↑

7s↑↓

Discovery and name

Discovered

1944 — Glenn T. Seaborg, Ralph A. James, Leon O. Morgan and Albert Ghiorso · United States (Chicago).

Name

The Americas (by analogy with europium).

Known for

Americium-241 is in most household ionisation smoke detectors — a tiny source that ionises air in a sensing chamber.

Isotopes of americium

Americium has no stable isotopes and does not occur naturally in significant amounts. Its longest-lived known isotope is:

Isotope	Protons	Neutrons	Half-life
²⁴³Am	95	148	7.35 thousand years

27 isotopes of americium are known in our data set (half-lives as published in nuclear data tables).

Americium and its neighbours in the actinide series

Neighbouring elements in the same f-block series:

Element	Z	Atomic weight	Melting point	Boiling point	Density	Electronegativity
Uranium	92	238.03	1,135 °C	4,131 °C	19.1 g/cm³	1.38
Neptunium	93	[237]	644 °C	—	20.2 g/cm³	1.36
Plutonium	94	[244]	640 °C	3,228 °C	19.7 g/cm³	1.28
Americium	95	[243]	1,176 °C	—	12 g/cm³	1.3
Curium	96	[247]	1,345 °C	—	13.51 g/cm³	1.3
Berkelium	97	[247]	986 °C	—	14.78 g/cm³	1.3
Californium	98	[251]	900 °C	—	15.1 g/cm³	1.3

Frequently asked questions

6 answers

What is the atomic mass of americium?

Americium has no stable isotopes, so it has no standard atomic weight. Periodic tables show [243], the mass number of its most stable known isotope.

How many protons, neutrons and electrons does americium have?

A neutral americium atom has 95 protons and 95 electrons. Its most stable isotope, americium-243, has 148 neutrons.

What is the electron configuration of americium?

[Rn] 5f⁷ 7s² (in full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f⁷ 6s² 6p⁶ 7s²).

What are the melting and boiling points of americium?

Americium melts at 1,176 °C (2,148.8 °F).

Is americium a metal?

Americium is a metal. It is in the f-block, period 7, and is a solid at room temperature.

Who discovered americium?

1944 — Glenn T. Seaborg, Ralph A. James, Leon O. Morgan and Albert Ghiorso (United States (Chicago)).